Recently, demand of an IC card is rising as an electronic approval card, a prepaid card, a periodic ticket and a card for individual certification and so on.
The IC card has the following feature.
1) The memory capacity of the IC card (500-16000 characters) is greater than the memory capacity of a conventional magnetic card (about 80 characters).
2) The IC card is harder to forge than the magnetic card.
3) By using the electromagnetic induction phenomenon by radio transmission, the nontouch type IC card can be manufactured.
Usually, a card( IC card, magnetic card) is put in a purse, a card case or carried loose; so, the IC card is required to satisfy the following same specifications compared to a magnetic card.
1) The size of the IC card is smaller than the magnetic card.
2) The thickness of the IC card is the same or thinner than the magnetic card.
3) The weight of the IC card is the same or lighter than the magnetic card.
4) The IC card is flexible against bending by external force.
The IC card has the IC chip, therefore, the IC card has a possibility of damage of the IC chip by the transformation.
The following is important in order to improve the reliability of the IC card against the transformation.
1) Structure of the IC card that resists the transformation to the IC chip.
2) Structure of the IC chip that even if the IC chip is transformed, the IC chip does not destroy.
The IC card that is hard to transform the IC chip even if the IC card is transformed is disclosed to Japanese patent Laid-open print No. 64-11895(prior art 1). Prior art 1 is disclosing the IC card structure that the IC chip suppresses the transformation by holding and reinforcing the IC chip with a metal plate.
It is important that the IC chip itself should be thin and to decrease the defect of the IC chip surface so that it is hard to destroy it by the transformation. A bending transformation of the IC chip and a stress of the IC chip surface are proportional to the thickness of the IC chip.
Generally Silicon (Si) used as a material of the IC chip is a brittle material. The IC chip is destroyed from the mirror surface defect. So, it is desirable to decrease the defect of the chip mirror surface.
A method of preventing destruction by forming the IC chip itself is disclosed to the Japanese patent Laid-open print No. 1-235699(prior art 2). Prior art 2 is disclosing the manufacturing method of removing the micro crack on the mirror surface of the IC chip by etching processing.
The Japanese patent Laid-open print No. 4-62921(prior art 3) uses plasma etching machine and is disclosing a removal of the reaction secondary product by 2 step etching.
The Japanese patent Laid-open print No. 6-232255(prior art 4) is dicing a wafer from the mirror surface of the chip to prevent the chipping that occurs in the mirror surface of the chip.
By the improvement of the manufacturing technique of the IC circuit, the thin type and the small area of the IC chip are advancing recent years. The thin type and the small area of the IC chip improve the flexibility against the transformation of the IC card. But high precision of the manufacture technique of the IC chip is also high is required. Despite the IC chip becoming thinner than the past and the removal the micro crack in the IC chip by etching before dicing wafer (prior art 2), the strength improvement of the chip is insufficient. The method of dicing of the wafer from the mirror surface of the chip (like prior art 4) cannot suppress the chipping that occurs in the device surface of the chip. Therefore, the IC chip strength is not improved.
The IC chip after dicing is thin, and the area is small. Therefore, it is difficult to etch only the mirror surface of the IC chip selectively by plasma etching, etc. from the problem of productivity and the transfer. When using isotropical chemical etching, etc., the device surface of the IC chip is eroded by etching. The purpose of the present invention is to provide an IC card which has high reliability against transformation by external force.
The inventors determined a break stress on the mirror surface and the device surface of the IC chip by 3 points bending test. The test result was analyzed by accumulative probability to destroy and break stress (FIG. 3). FIG. 3 (A) illustrates the break stress of the mirror surface of the IC chip. FIG.3 (B) illustrates the break stress of the device surface of the IC chip. As is clear from this figure, break stress on the mirror surface of the IC chip is smaller than break stress on the device surface of the IC chip and the dispersion of break stress also enlarges.
From this, it is thought that the defect much exists on the mirror surface of the IC chip. The inventors measured the break stress on the mirror surface of various IC chips for which the grinding conditions at the time of IC chip manufacturing are the same and dicing conditions are different, by the 3 points bending test. The result was arranged on FIG. 4. As it is shown in FIG. 4, dicing conditions (four conditions; Dicing A, Dicing B, Dicing C, Dicing D ) also greatly changes break stress of the IC chip. It is proved that the defect that arises from this dicing in the manufacturing process of the IC chip has given the break strength of the IC chip a large influence.
In FIG. 5, the typeof defects on the mirror surface after dicing the IC chip are shown. The circumference part of the chip is inhere many flaws occur. This is chipping caused on the mirror surface of the IC card by the process of dicing. From this, the destruction strength of the IC chip declines by the chipping that arises on the mirror surface side by dicing of the manufacturing process of the IC chip.
That is, from the inventor""s analysis it is found that elimination of the chipping that exists in the circumference part on the mirror surface of the IC chip improves the destruction strength of the IC chip. The process of etching the IC chip is done after dicing to eliminate the chipping that occurs on the mirror surface of the IC chip. But the IC chip that is the present application field of the invention has a thickness 60 xcexcm or less. The IC chip, after dicing is very thin and small so it is difficult to handle. Therefore, there is a problem in productivity and the transfer by the method of etching only the mirror surface selectively by plasma etching machine, etc.
When the IC chip is etched by chemical etching, the problem of the above productivity and the transfer does not arise. But a problem has been discovered of eroding the device surface as well as the mirror surface of the IC chip by etching because etching cannot be done selectively.